cmd/compile: add types.SoleComponent, use in convFuncName

The specialized conversion functions care only
about a type's layout in memory, so e.g.
[1]string is equivalent to string.

Add types.SoleComponent to assist with such use cases,
and use it for the specialized conversion functions.

Increases the number of convTstring calls by ~1%.

Change-Id: I09a392909f2037387b30642781e65f707a048af5
Reviewed-on: https://go-review.googlesource.com/c/148577


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>